Workers in the field of integrated circuits have long fabricated thin-film resistors in the back end of an integrated circuit. In contrast to resistors located in the silicon substrate of the circuit, which suffer from relatively large parasitic capacitance to the substrate, back end resistors have much less parasitic capacitance, since they are placed vertically above the substrate. Hence, back end of the line (BEOL) resistors are ideal for high frequency RF circuitry applications.
BEOL thin-film resistors are deposited in layers of a material of known resistivity and having a well controlled thickness and therefore inherently have more accurate resistance values than front end resistors that are implanted in the substrate or polysilicon layer and annealed subsequently using rapid thermal annealing processing. The accuracy of the front end resistor is limited due to the inherit variations in thermal temperature control and ion implantation processing.
It is not enough that the resistor is fabricated with the benefits recited above, however. The process of forming the resistor must be integrated into the overall back end process. If the resistor had to be formed using a series of steps that were different from other steps in forming the back end, the cost of forming a resistor would be excessive.
In earlier and current work, e.g. the 180 nm node and above, integration of thin film resistors was straightforward. As the technology advances to the 90 nm node and then to the 65 nm and 45 nm nodes, however, integration will become more difficult because the thickness of the layers in the back end will decrease, making integration progressively more difficult.
The art could benefit from an integrated process for forming a thin-film resistor in the back end of an integrated circuit that is effective for thin layers of interlevel dielectric (ILD) in the back end structure.